American imperialism is the concept of an American Empire was first popularized during the presidency of James K. Polk who led the United States into the Mexican–American War of 1846, and the eventual annexation of California and other western territories via the Treaty of Guadalupe Hidalgo and the Gadsden purchase.

American imperialism is the concept of an American Empire was first popularized during the presidency of James K. Polk who led the United States into the Mexican–American War of 1846, and the eventual annexation of California and other western territories via the Treaty of Guadalupe Hidalgo and the Gadsden purchase.